1. Field of the Invention
The present invention relates to a feedback circuit provided in a transistor and an amplifier and a mixer comprising the same.
2. Description of the Related Art
In recent years, radio waves having a great many frequencies have been required in order to establish communication with the rapid development of mobile communication. The frequencies of the radio waves used in the mobile communication have been shifted to a microwave band. A handy terminal for such mobile communication is provided with an amplifier using a field effect transistor (hereinafter referred to as xe2x80x9cFETxe2x80x9d). There is a method of providing a feedback circuit in one of the designs of the amplifier.
FIG. 26 is a circuit diagram of an amplifier comprising a conventional feedback circuit.
As shown in FIG. 26, a feedback circuit 200 comprising a resistor 101 and a capacitor 102 is connected between a drain electrode (an output terminal) and a gate electrode (an input terminal) of an FET 100. A signal outputted from the drain electrode of the FET 100 is fed back to the gate electrode in reversed phase by the feedback circuit 200. Consequently, the oscillation of the FET 100 is prevented by a negative feedback effect, and the gain of the FET 100 is adjusted.
On the other hand, in a multi-channel communication system, a wide band amplifier is used to simultaneously amplify a plurality of carrier waves. Generally, when the linearity of the amplifier is degraded, several harmonics such as a second harmonic (second order harmonic) and a third harmonic (third order harmonic) for a fundamental wave are generated. It has been known that the harmonics cause various disturbance waves such as intermodulation distortions.
In the intermodulation distortions, a second order distortion (IM2) is generated mainly as a result of the second harmonic, and third order distortion (IM3) is generated mainly as a result of the third harmonic. Further, in the multi-channel communication system, composite second order distortions (CSO) are generated by the second order distortion on a plurality of channels, and composite triple beat (CTB) and cross modulation (X-MOD) are generated by the third order distortion on the plurality of channels.
When a fundamental wave s1 having a frequency f1 and a fundamental wave s2 having a frequency f2 are inputted to the amplifier, as shown in FIG. 27, for example, disturbance waves im2 respectively having a frequency (f1+f2) and a frequency (f2xe2x88x92f1) are generated by the composite second order distortions. Further, disturbance waves im3 respectively having a frequency (2f2xe2x88x92f1) and a frequency (2f1xe2x88x92f2) are generated by the composite third order distortions.
In the case of a cable television system, 100 or more channels exist at spacing of 6 MHz in a band from approximately 100 to 800 MHz. When a fundamental wave having a frequency of 750 MHz (f1) on a certain channel and a fundamental wave having a frequency of 756 MHz (f2) on the other channel, for example, are inputted, disturbance waves are respectively generated at a frequency of 744 MHz (2f1xe2x88x92f2) and a frequency of 762 MHz (2f2xe2x88x92f1) in the band due to the composite third order distortions. The respective frequencies of the disturbance waves are the same as the frequency of the adjacent channel. Accordingly, the fundamental wave on the adjacent channel is disturbed by the disturbance waves.
When the fundamental wave having a frequency of 750 MHz (f1) on a certain channel and the fundamental wave having a frequency of 450 MHz (f2) on the other channel are inputted, a disturbance wave is generated at a frequency of 300 MHz (f2xe2x88x92f1) in the band as a result of the composite second order distortions. The frequency of the disturbance wave is the same as the frequency of the other channel. Accordingly, the fundamental wave of the other channel is disturbed.
Consequently, it is required that a wide band amplifier for simultaneously amplifying a plurality of carrier waves has high linearity in order to prevent multi-channel intermodulation distortions. For example, JP-A-10-290125 discloses a power amplifier having a nonlinear distortion compensating circuit of a feedforward type.
In the power amplifier, in the entire band, an inputted signal is distributed into signals. One of the signals obtained by the distribution is separated into bands requiring nonlinear distortion compensation. A distortion component and the amount of compensation in each of the bands are detected, to synthesize compensating signals. The compensating signal is used, to compensate for the other signal obtained by the distribution.
In the conventional amplifier shown in FIG. 26, the feedback circuit 200 is constituted by the resistor 101 and the capacitor 102. Even when the resistance value of the resistor 101 and the capacitance value of the capacitor 102 are set such that a desired frequency is negatively fed back, therefore, not only the desired frequency but also all frequencies are negatively fed back. Particularly, the feedback circuit 200 has the capacitor 102. When the frequency is increased, therefore, the impedance is decreased. Accordingly, the higher the order of a harmonic is than that of the second harmonic, the larger the amount of feedback thereof is. That is, it is impossible to perform setting such that a particular frequency is not fed back.
If the setting is performed such that a second harmonic having a frequency which is two times the frequency of the fundamental wave or a harmonic of a higher order than the second harmonic is fed back, the fundamental wave is fed back to some extent. As a result, the characteristics of the amplifier and particularly, the gain thereof is degraded. A matching circuit for impedance matching with a load circuit connected to the amplifier is difficult to design.
On the other hand, in the power amplifier disclosed in JP-A-10-290125, several circuits such as a circuit for distributing a signal in an input portion, a circuit for separating each of signals obtained by the distribution into bands, a circuit for detecting a distortion component and the amount of compensation in each of the bands, and a circuit for synthesizing compensating signals must be provided. Accordingly, the circuit scale is significantly increased, and the power consumption in the circuits is also increased.
It is necessary to determine a pass-band of a band-pass filter in conformity with the bandwidth of a channel in a communication system using the power amplifier. Consequently, the power amplifier designed for a certain communication system cannot be applied to another communication system which differs in the channel bandwidth.
An object of the present invention is to provide a feedback circuit capable of reducing nonlinear distortion generated in a transistor on a small circuit scale.
Another object of the present invention is to provide an amplifier comprising a feedback circuit capable of reducing nonlinear distortion generated in a transistor on a small circuit scale.
Still another object of the present invention is to provide a mixer comprising a feedback circuit capable of reducing nonlinear distortion generated in a transistor on a small circuit scale.
A feedback circuit connected between an output terminal and an input terminal of a transistor according to one aspect of the present invention comprises at least one series connection circuit comprising a series connection of a capacitance and an inductance, the value of the capacitance and the value of the inductance of the at least one series connection circuit being set such that the at least one series connection circuit enters a substantially opened state with respect to a first frequency, and the at least one series connection circuit enters a substantially short-circuited state with respect to a second frequency.
In the feedback circuit, the series connection circuit including the series connection of the capacitance and the inductance enters the substantially opened state with respect to the first frequency and enters the substantially short-circuited state with respect to the second frequency. Consequently, a component having the second frequency of a signal outputted from the output terminal of the transistor is negatively fed back to the input terminal, and the component having the second frequency inputted to the input terminal is canceled by a component having the second frequency negatively fed back.
As a result, it is possible to reduce nonlinear distortion generated by the component having the second frequency without decreasing the gain for the component having the first frequency. Further, the feedback circuit is constituted by the series connection circuit including the capacitance and the inductance, so that the circuit scale is small.
The fact that the series connection circuit enters the substantially opened state with respect to the first frequency and enters the substantially short-circuited state with respect to the second frequency means that the impedance of the series connection circuit increases to infinity or a sufficiently high value with respect to the first frequency, and reduces to zero or a sufficiently low value with respect to the second frequency.
The at least one series connection circuit may include a plurality of series connection circuits, the plurality of series connection circuits may be connected in parallel between the output terminal and the input terminal of the transistor, the values of the capacitances and the values of the inductances of the plurality of series connection circuits being set such that each of the series connection circuits enters a substantially opened state with respect to the first frequency, and the series connection circuits enter a substantially short-circuited state, respectively, with respect to different second frequencies.
In the feedback circuit, the plurality of series connection circuits each comprising the series connection of the capacitance and the inductance enters the substantially opened state with respect to the first frequency, and the series connection circuits enter the substantially short-circuited state, respectively, with respect to the different second frequencies. Consequently, components having the plurality of different second frequencies of a signal outputted from the output terminal of the transistor are negatively fed back to the input terminal, and components having the plurality second frequencies inputted to the input terminal are respectively canceled by the components having the second frequencies negatively fed back.
As a result, it is possible to reduce nonlinear distortion generated by the components having the plurality of different second frequencies without decreasing the gain for the component having the first frequency. Further, the feedback circuit is constituted by the plurality of series connection circuits each comprising the series connection of the capacitance and the inductance, so that the circuit scale is small.
The first frequency may be the frequency of a fundamental wave, and the second frequency may be the frequency of a harmonic corresponding to the fundamental wave. The second frequency may be the frequency of a third harmonic or a fifth harmonic corresponding to the fundamental wave.
In this case, the harmonic outputted from the output terminal of the transistor is negatively fed back to the input terminal, and the harmonic inputted to the input terminal is canceled by the harmonic negatively fed back. Consequently, the harmonic is prevented without decreasing the gain for the fundamental wave. As a result, it is possible to reduce nonlinear distortion generated by the harmonic.
The first frequency may include parts or all of frequencies in a band corresponding to a communication system, or may include a frequency most adjacent to the band corresponding to the communication system. Further, the second frequency is a harmonic having the first frequency, and may include a frequency which is an integral multiple of the frequency in the band including the first frequency.
The second frequency may be the frequency of a fundamental wave, and the first frequency may be the frequency of a harmonic corresponding to the fundamental wave. The first frequency may be the frequency of a third harmonic or a fifth harmonic corresponding to the fundamental wave.
In this case, the fundamental wave of the signal outputted from the output terminal of the transistor is negatively fed back to the input terminal. Consequently, the oscillation of the transistor at the frequency of the fundamental wave is prevented. As a result, the transistor is stabilized.
The feedback circuit may further comprise a feedback amount adjusting resistor connected in series with the at least one series connection circuit.
In this case, the amount of feedback power of the component having the second frequency from the output terminal to the input terminal of the transistor can be adjusted by adjusting the value of the feedback amount adjusting resistor.
A feedback circuit connected between an output terminal and an input terminal of a transistor according to another aspect of the present invention further comprises a high pass filter having the property of preventing a component having a first frequency and passing a component having a second frequency higher than the first frequency.
In the feedback circuit, the high pass filter prevents the component having the first frequency and passes the component having the second frequency higher than the first frequency. Consequently, the component having the second frequency of the signal outputted from the output terminal of the transistor is negatively fed back to the input terminal, and the component having the second frequency inputted to the input terminal is canceled by the component having the second frequency negatively fed back.
As a result, it is possible to reduce nonlinear distortion generated by the component having the second frequency without decreasing the gain for the component having the first frequency. Further, the feedback circuit is constituted by the high pass filter, so that the circuit scale is small.
It is preferable that the first frequency is included in a band including a channel to be used, and the cut-off frequency of the high pass filter is higher than the highest frequency in the band including the channel.
In this case, the component having the first frequency included in the band including the channel of the signal outputted from the output terminal of the transistor is not negatively fed back to the input terminal, and the component having the frequency higher than the highest frequency in the band including the channel is negatively fed back to the input terminal. Consequently, at least one harmonic corresponding to at least one fundamental wave included in the band including the channel is negatively fed back, and the harmonic inputted to the input terminal is canceled by the harmonic negatively fed back. As a result, it is possible to reduce nonlinear distortion generated by the harmonic without decreasing the gain for the fundamental wave included in the band including the channel.
It is preferable that the cut-off frequency of the high pass filter is not more than three times the highest frequency in the band including the channel.
In this case, at least the component having the frequency which is higher than the highest frequency in the band including the channel and is not more than three times the highest frequency of the signal outputted from the output terminal of the transistor is negatively fed back to the input terminal. Consequently, the hither harmonic on the third or more order with respect to at least one fundamental wave in the band including the channel is negatively fed back, and the harmonic on the third or more order which is inputted to the input terminal is canceled by the harmonic on the third or more order negatively fed back. As a result, it is possible to reduce nonlinear distortion generated by the harmonic on the third or more order.
The feedback circuit may further comprise a feedback amount adjusting resistor connected in series with the high pass filter.
In this case, the value of the feedback amount adjusting resistor is adjusted, thereby making it possible to adjust the amount of feedback power of the component having the second frequency from the output terminal to the input terminal of the transistor.
An amplifier according to still another aspect of the present invention comprises a transistor; and a feedback circuit connected between an output terminal and an input terminal of the transistor, the feedback circuit comprising at least one series connection circuit comprising a series connection of a capacitance and an inductance, the value of the capacitance and the value of the inductance of the at least one series connection circuit being set such that the at least one series connection circuit enters a substantially opened state with respect to a first frequency, and the at least one series connection circuit enters a substantially short-circuited state with respect to a second frequency.
In the amplifier, the feedback circuit is connected between the output terminal and the input terminal of the transistor. Accordingly, it is possible to reduce nonlinear distortion generated by a component having the second frequency without decreasing the gain for a component having the first frequency. Further, the circuit scale of the feedback circuit is small, thereby making it possible to miniaturize the amplifier.
The at least one series connection circuit may include a plurality of series connection circuits, the values of the capacitances and the values of the inductances of the plurality of series connection circuits being set such that each of the series connection circuits enters a substantially opened state with respect to the first frequency, and the series connection circuits enter a substantially short-circuited state, respectively, with respect to different second frequencies.
In this case, it is possible to reduce the nonlinear distortion generated by components having the plurality of different second frequencies without decreasing the gain for a component having the first frequency. Further, the feedback circuit is constituted by the plurality of series connection circuits each comprising the series connection of the capacitance and the inductance, so that the circuit scale is small.
An amplifier according to still another aspect of the present invention comprises a transistor; and a feedback circuit connected between an output terminal and an input terminal of the transistor, the feedback circuit comprising a high pass filter having the property of preventing a component having a first frequency and passing a component having a second frequency higher than the first frequency.
In the amplifier, the feedback circuit is connected between the output terminal and the input terminal of the transistor. Accordingly, it is possible to reduce nonlinear distortion generated by the component having the second frequency without decreasing the gain for the component having the first frequency. Further, the feedback circuit is constituted by the high pass filter, so that the circuit scale is small.
It is preferable that the first frequency is included in a band including a channel to be used, and the cut-off frequency of the high pass filter is higher than the highest frequency in the band including the channel.
In this case, the component having the first frequency included in the band including the channel of a signal outputted from the output terminal of the transistor is not negatively fed back to the input terminal, and the component having the frequency higher than the highest frequency in the band including the channel is negatively fed back. Consequently, at least one harmonic with respect to at least one fundamental wave included in the band including the channel is negatively fed back, and the harmonic inputted to the input terminal is canceled by the harmonic negatively fed back. As a result, it is possible to reduce nonlinear distortion generated by the harmonic without decreasing the gain for the fundamental wave included in the band including the channel.
A mixer according to still another aspect of the present invention comprises a transistor; and a feedback circuit connected between an output terminal and an input terminal of the transistor, the feedback circuit comprising at least one series connection circuit comprising a series connection of a capacitance and an inductance, the at least one series connection circuit being connected between the output terminal and the input terminal of the transistor, the value of the capacitance and the value of the inductance of the at least one series connection circuit being set such that the at least one series connection circuit enters a substantially opened state with respect to a first frequency, and the at least one series connection circuit enters a substantially short-circuited state with respect to a second frequency.
In the mixer, the feedback circuit is connected between the output terminal and the input terminal of the transistor. Accordingly, it is possible to reduce nonlinear distortion generated by a component having the second frequency without decreasing the gain for a component having the first frequency. Further, it is possible to reduce the nonlinear distortion generated by the component having the second frequency. Further, the circuit scale of the feedback circuit is small, thereby making it possible to miniaturize the mixer.
The first frequency may include parts or all of frequencies in a band corresponding to a communication system. When the second frequency is the harmonic having the first frequency, the second frequency may include a frequency which is an integral multiple of the frequency in the band including the first frequency.
The transistor may be a field effect transistor having first and second gate electrodes respectively receiving first and second high-frequency signals.
In this case, the first high-frequency signal fed to the first gate electrode and the second high-frequency signal fed to the second gate electrode are mixed with each other.
A mixer according to still another aspect of the present invention comprises a transistor; and a feedback circuit connected between an output terminal and an input terminal of the transistor, the feedback circuit comprising a high pass filter having the property of preventing a component having a first frequency and passing a component having a second frequency higher than the first frequency.
In the mixer, the feedback circuit is connected between the output terminal and the input terminal of the transistor. Accordingly, it is possible to reduce nonlinear distortion generated by the component having the second frequency without decreasing the gain for the component having the first frequency. Further, the feedback circuit is constituted by the high pass filter, so that the circuit scale is small.
The transistor is a field effect transistor having first and second gate electrodes respectively receiving first and second high-frequency signals.
In this case, the first high frequency signal fed to the first gate electrode and the second high frequency signal fed to the second gate electrode are mixed with each other.
It is preferable that the first frequency is included in a band including a channel to be used, and the cut-off frequency of the high pass filter is higher than the highest frequency in the band including the channel.
In this case, the component having the first frequency included in the band including the channel of a signal outputted from the output terminal of the transistor is not negatively fed back to the input terminal, and the component having the frequency higher than the highest frequency in the band including the channel is negatively fed back. Consequently, at least one harmonic corresponding to at least one fundamental wave included in the band including the channel is negatively fed back, and the harmonic inputted to the input terminal is canceled by the harmonic negatively fed back. As a result, it is possible to reduce nonlinear distortion generated by the harmonic without decreasing the gain for the fundamental wave included in the band including the channel.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.